The present invention relates to sensors for visual range and rain-covered area detection, particularly for use in a motor vehicle.
Three principles distinguish the measuring of visual range: contrast, absorption and reflection measurements. The first principle requires an object in front of a black background which is marked by defined brightness. If this object can still be distinguished by the human eye, it is within visual range. This principle can be automated with the aid of photo and video technology, except for mobile use, since no suitable object for contrast can be guaranteed. The second principle is used at highway measuring points. Here a light beam which is sent from one side (generally with invisible infrared light) is received by the opposite side approximately 2 m away. In fog the light reception power drops. The drop then becomes a measure for the visual range. For self-sufficient detection in a moving motor vehicle this principle too is practically useless. In this case, the reflection principle could be applied, which is however much weaker. It is based on the back scatter of emitted light from fog drops. In fog this effect leads also to the well-known blinding effects created by the high beams. Reflection measurement is however sufficient for at least a rough classification of visual range. Principally a simple measuring arrangement that works like a reflecting light barrier is sufficient for the measurement. Although the results can be very easily interfered with and depend upon many parameters.
A marked improvement of robustness can be achieved via a locally dissolving back scatter measurement, in which the distance-dependent back scatter distribution can be utilized for visual range detection. Such a measuring procedure is the measuring of back scatter in several distance cells. These become distinguishable through correlation technologies. This measuring procedure corresponds to the common LIDAR (Light Detecting and Ranging) sensors, which are designated as distance sensors for distance warning and ACC. LIDAR sensors lend themselves well visual range detection. But particularly the back scatter under fog conditions leads to the biggest disadvantage of these sensors, in that the detection range is severely reduced.
An alternative procedure for determining the distance is the triangulation technology by which the transmitter and/or receiver is separated by a so-called base width. The strongly pointed transmitter and receiver lobes overlap with each other in a defined area, so that the reflection can only be detected from this area. Several transmitter and receiver lobes thus allow inclusion of several back scatter areas.
A particular difficulty with simple visual range detection is that it is hard to distinguish whether good visual range exists or whether important sensor components failed. Indirect testing can be done by testing the transmitter or receiver by means of comparing one or several reference light channels, which allows detection of a large part of possible defects.
Currently the most promising procedure for rain-covered area detection is based on decoupling a light beam from a light channel, which is limited by the edge surface of the windshield. These edge surfaces reflect the light channeled in by means of a prism, since the light beam angle is too small to allow for transmission. If a rain drop now moistens the light channel, the condition for reflection (windshield--air) then ceases, but instead a large part of the light is emitted from this drop. The decreasing light capacity of the channel is measured at the decoupling point (again prism or similar) by means of photo diodes or photo transistors. For synchronized light and noise suppression the transmission power is generally modulated and the receiver signal is measured in a phase sensitive manner.